Tactile allodynia in the absence of C-fiber activation: altered firing properties of DRG neurons following spinal nerve injury

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Behavioral and electrophysiological studies in rats with cisplatin-induced chemoneuropathy

Neuropathy is the chief dose-limiting side effect associated with the major classes of frontline cancer therapy drugs. Here the changes in behavioral responses of rats to cutaneous mechanical and thermal stimuli occurring following treatment with cisplatin and the changes in spinal neurophysiology accompanying the development of chemotherapy-induced hyperalgesia were explored. Systemic treatment with cisplatin induced changes in both mechanical and thermal cutaneous sensory withdrawal thresholds of Sprague-Dawley rats. High doses of chemotherapy produced hypoalgesia whereas lower doses produced hyperalgesia. Follow-up neurophysiological studies in rats with chemotherapy-induced hyperalgesia revealed that deep spinal lamina wide dynamic range neurons had significantly higher spontaneous activity and longer afterdischarges to noxious mechanical stimuli than wide dynamic range neurons in control rats; cisplatin administration was also associated with longer afterdischarges and abnormal wind-up to transcutaneous electrical stimuli. The hyperexcitability observed during cisplatin-induced hyperalgesia is very similar to that observed in rats with hyperalgesia produced following treatment with other very diverse types of chemotherapeutic agents and similar to that observed following specific types of direct nerve injury.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Characteristics of HCN channels and their participation in neuropathic pain

Neuropathic pain is induced by the injury to nervous systems and characterized by hyperalgesia, allodynia and spontaneous pain. The underlying mechanisms include peripheral and central sensitization resulted from neuronal hyperexcitability. A number of ion channels are considered to contribute to the neuronal hyperexcitability. Here, we particularly concentrate on an interesting ion channel, hyperpolarization-activated cyclic nucleotide gated (HCN) channels. We overview its biophysical properties, physiological functions, followed by focusing on the current progress in the study of its role in the development of neuropathic pain. We attempt to provide a comprehensive review of the potential valuable target, HCN channels, in the treatment of neuropathic pain.

Effects of carbamazepine and amitriptyline on tetrodotoxinresistant Na+ channels in immature rat trigeminal ganglion neurons

Although anticonvulsant drugs that block voltage-dependent Na+ channels have been widely used for neuropathic pain, including peripheral nerve injury-induced pain, much less is known about the actions of these drugs on immature trigeminal ganglion (TG) neurons. Here we report the effects of carbamazepine (CBZ) and amitriptyline (ATL) on tetrodotoxin-resistant (TTX-R) Na' channels expressed on immature rat TG neurons. TTX-R Na+ currents (I(Na)) were recorded in the presence of 300 nM TTX by use of a conventional whole-cell patch clamp method. Both CBZ and ATL inhibited TTX-R I(Na) in a concentration-dependent manner, but ATL was more potent. While CBZ and ATL did not affect the overall voltage-activation relationship of TTX-R Na+ channels, both drugs shifted the voltage-activation relationship to the left, indicating that they inhibited TTX-R Na+ channels more efficiently at depolarized membrane potentials. ATL showed a profound use-dependent blockade of TTX-R I(Na), but CBZ had little effect. The present results suggest that both CBZ and ATL, common drugs used for treating neuropathic pain, efficiently inhibit TTX-R Na+ channels expressed on immature TG neurons, and that these drugs might be useful for the treatment of trigeminal nerve injury-induced neuropathic pain, as well as the inhibition of ongoing central sensitization, even during immature periods.

Multiple sites for generation of ectopic spontaneous activity in neurons of the chronically compressed dorsal root ganglion

In a chronically compressed dorsal root ganglion (CCD) in the rat, a model of foraminal stenosis and radicular pain in human, a subpopulation of neurons with functional axons exhibits spontaneous activity (SA) that originates within the ganglion. Intracellular electrophysiological recordings were obtained from the somata of neurons of the compressed ganglion both in vitro and in vivo. The SA was classified into two types according to the presence (type I) or absence (type II) of subthreshold membrane potential oscillation. Neurons with type I SA had significantly higher somal excitability than those with type II SA. In most cases, depolarization of the membrane potential by current injection increased the discharge rates of type I--but not type II SA. Both types occasionally coexisted in the same neuron. Several lines of evidence suggested that the origin of SA in the DRG was most likely the soma for type I SA and the axon for type II. Therefore CCD neurons have multiple sites for generation of action potentials other than the terminal endings. In vivo recordings revealed the same two types of SA in a subpopulation of neurons with functionally characterized peripheral receptive fields. Thus, SA might not only produce spurious sensory input to the afferent pathways but also add to or block impulse transmission generated by natural stimulation of peripheral receptors. SA originating in the compressed ganglion is likely to interfere with sensory transmission in nociceptive and non-nociceptive neurons, thereby contributing to radicular pain, paresthesias, hyperalgesia and allodynia.

Axonal accumulation of hyperpolarization-activated cyclic nucleotide-gated cation channels contributes to mechanical allodynia after peripheral nerve injury in rat

Peripheral nerve injury causes neuropathic pain including mechanical allodynia and thermal hyperalgesia due to central and peripheral sensitization. Spontaneous ectopic discharges derived from dorsal root ganglion (DRG) neurons and from the sites of injury are a key factor in the initiation of this sensitization. Numerous studies have focused primarily on DRG neurons; however, the injured axons themselves likely play an equally important role. Previous studies of neuropathic pain rats with spinal nerve ligation (SNL) showed that the hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel in DRG neuronal bodies is important for the development of neuropathic pain. Here, we investigate the role of the axonal HCN channel in neuropathic pain rats. Using the chronic constriction injury (CCI) model, we found abundant axonal accumulation of HCN channel protein at the injured sites accompanied by a slight decrease in DRG neuronal bodies. The function of these accumulated channels was verified by local application of ZD7288, a specific HCN blocker, which significantly suppressed the ectopic discharges from injured nerve fibers with no effect on impulse conduction. Moreover, mechanical allodynia, but not thermal hyperalgesia, was relieved significantly by ZD7288. These results suggest that axonal HCN channel accumulation plays an important role in ectopic discharges from injured spinal nerves and contributes to the development of mechanical allodynia in neuropathic pain rats.

The antipsychotic drug, fluphenazine, effectively reverses mechanical allodynia in rat models of neuropathic pain

RATIONALE

Fluphenazine is a potent antipsychotic drug used to treat schizophrenia and other psychotic symptoms. Its clinical benefit is mainly mediated by the antagonism of dopamine D2 receptors. We have recently discovered, however, that fluphenazine is also a potent sodium channel blocker, a property that may offer additional therapeutical indications, including analgesia.

OBJECTIVES

The present study sought to determine the analgesic effect of fluphenazine on neuropathic pain in animal models.

METHODS

The effect of fluphenazine on mechanical allodynia was assessed in three animal neuropathic pain models, including spinal nerve ligation, chronic constriction nerve injury (CCI), and sural-spared sciatic nerve injury models.

RESULTS

Systemic fluphenazine effectively attenuated mechanical allodynia in all three rat neuropathic pain models at doses (0.03-0.3 mg/kg) that approximate those used in rodent models of psychosis. In parallel with its in vivo antiallodynic effect, fluphenazine (3-30 microM) effectively suppressed the ectopic discharges in injured afferent fibers without affecting the propagation of action potentials evoked by electrical nerve stimulation in an ex vivo dorsal root ganglia (DRG)-nerve preparation excised from CCI rats. Furthermore, similar concentrations of fluphenazine significantly blocked sodium channels in DRG neurons.

CONCLUSIONS

The inhibitory action of fluphenazine on ectopic afferent discharges may be due to its ability to block voltage-gated sodium channels, and this may also provide a mechanistic basis for the drug's antiallodynic effect in animal models of neuropathic pain. In summary, our study demonstrates that the classic antipsychotic drug fluphenazine has antiallodynic properties in multiple rodent models of nerve injury-induced neuropathic pain.

Delayed functional expression of neuronal chemokine receptors following focal nerve demyelination in the rat: a mechanism for the development of chronic sensitization of peripheral nociceptors

BACKGROUND

Animal and clinical studies have revealed that focal peripheral nerve axon demyelination is accompanied by nociceptive pain behavior. C-C and C-X-C chemokines and their receptors have been strongly implicated in demyelinating polyneuropathies and persistent pain syndromes. Herein, we studied the degree to which chronic nociceptive pain behavior is correlated with the neuronal expression of chemokines and their receptors following unilateral lysophosphatidylcholine (LPC)-induced focal demyelination of the sciatic nerve in rats.

RESULTS

Focal nerve demyelination increased behavioral reflex responsiveness to mechanical stimuli between postoperative day (POD) 3 and POD28 in both the hindpaw ipsilateral and contralateral to the nerve injury. This behavior was accompanied by a bilateral increase in the numbers of primary sensory neurons expressing the chemokine receptors CCR2, CCR5, and CXCR4 by POD14, with no change in the pattern of CXCR3 expression. Significant increases in the numbers of neurons expressing the chemokines monocyte chemoattractant protein-1 (MCP-1/CCL2), Regulated on Activation, Normal T Expressed and Secreted (RANTES/CCL5) and interferon gamma-inducing protein-10 (IP-10/CXCL10) were also evident following nerve injury, although neuronal expression pattern of stromal cell derived factor-1alpha (SDF1/CXCL12) did not change. Functional studies demonstrated that acutely dissociated sensory neurons derived from LPC-injured animals responded with increased [Ca2+]i following exposure to MCP-1, IP-10, SDF1 and RANTES on POD 14 and 28, but these responses were largely absent by POD35. On days 14 and 28, rats received either saline or a CCR2 receptor antagonist isomer (CCR2 RA-[R]) or its inactive enantiomer (CCR2 RA-[S]) by intraperitoneal (i.p.) injection. CCR2 RA-[R] treatment of nerve-injured rats produced stereospecific bilateral reversal of tactile hyperalgesia.

CONCLUSION

These results suggest that the presence of chemokine signaling by both injured and adjacent, uninjured sensory neurons is correlated with the maintenance phase of a persistent pain state, suggesting that chemokine receptor antagonists may be an important therapeutic intervention for chronic pain.

Evaluation of the enantioselective antiallodynic and pharmacokinetic profile of propylisopropylacetamide, a chiral isomer of valproic acid amide

Propylisopropylacetamide (PID) is a chiral CNS-active constitutional isomer of valpromide, the amide derivative of the major antiepileptic drug valproic acid (VPA). The purpose of this work was: a) To evaluate enantiospecific activity of PID on tactile allodynia in the Chung (spinal nerve ligation, SNL) model of neuropathic pain in rats; b) To evaluate possible sedation at effective antiallodynic doses, using the rotorod ataxia test; c) To investigate enantioselectivity in the pharmacokinetics of (R)- and (S)-PID in comparison to (R,S)-PID; and d) To determine electrophysiologically whether PID has the potential to affect tactile allodynia by suppressing ectopic afferent discharge in the peripheral nervous system (PNS). (R)-, (S)- and (R,S)-PID produced dose-related reversal of tactile allodynia with ED(50) values of 46, 48, 42 mg/kg, respectively. The individual PID enantiomers were not enantioselective in their antiallodynic activity. No sedative side-effects were observed at these doses. Following i.p. administration of the individual enantiomers, (S)-PID had lower clearance (CL) and volume of distribution (V) and a shorter half-life (t(1/2)) than (R)-PID. However following administration of (R,S)-PID, both enantiomers had similar CL and V, but (R)-PID had a longer t(1/2). Systemic administration of (R,S)-PID at antiallodynic doses did not suppress spontaneous ectopic afferent discharge generated in the injured peripheral nerve, suggesting that its antiallodynic action is exerted in the CNS rather than the PNS. Both of PID's enantiomers, and the racemate, are more potent antiallodynic agents than VPA and have similar potency to gabapentin. Consequently, they have the potential to become new drugs for treating neuropathic pain.

Thalamic activation in a disc herniation model

STUDY DESIGN

A novel approach combining a rodent disc herniation model with electrophysiologic recordings of thalamic evoked responses.

OBJECTIVE

To assess short-term effects of nucleus pulposus (NP) application on dorsal root ganglions (DRG) on high threshold afferent fiber evoked activation in the thalamus.

SUMMARY OF BACKGROUND DATA

Epidural application of NP in combination with mechanical compression induces pain related behavior in rats associated with enhanced activity of pain-processing neurons in the dorsal horn of the spinal cord. However, possible effects on neuronal activity in the pain processing ventral posterior lateral (VPL) thalamic nucleus following NP application on DRG have not been investigated.

METHODS

Responses in the contralateral VPL evoked by electrical stimulation of the sciatic nerve and of the fourth lumbar (L4) DRG were recorded in adult Sprague-Dawley rats. Records were obtained before and during application (5, 10, and 20 minutes) of NP or of adipose tissue (AT) to the L4 DRG. AT was used as control for mechanical effects of NP application.

RESULTS

Application of NP resulted in an increase of evoked thalamic responses to 138% +/- 10% of control after 20 minutes (P < 0.01), whereas AT application for 20 minutes resulted in a reduction of evoked responses to 77% +/- 4% (P < 0.05). Recordings in control animals (i.e., with no application) demonstrated stable evoked neuronal thalamic activity for up to 40 minutes.

CONCLUSION

The study demonstrates that NP application onto DRG increases afferent fiber evoked responses in the thalamus and in view of the opposite effects of AT application suggests that these effects may be specific for NP. The results show that NP affects sensory transmitting pathways within a few minutes, possibly due to rapid and reversible alterations in the neuronal excitability. The study thus introduces a rodent model for studying sensory afferent evoked thalamic activity related to DRG injury which may be used to evaluate analgesics and anti-inflammatory drugs used for pain relief in disc herniation and neuropathic pain patients.

Plasticity in intact A delta- and C-fibers contributes to cold hypersensitivity in neuropathic rats

Cold hypersensitivity is a common sensory abnormality accompanying peripheral neuropathies and is difficult to treat. Progress has been made in understanding peripheral mechanisms underlying neuropathic pain but little is known concerning peripheral mechanisms of cold hypersensitivity. The aim of this study was to analyze the contribution of uninjured primary afferents to the cold hypersensitivity that develops in neuropathic rats. Rats with a lumbar 5 (L5) and L6 spinal nerve ligation (SNL, Chung model) but not sham, developed mechanical allodynia, evidenced by decreased paw withdrawal thresholds and increased magnitude of response to von Frey stimulation. Cold hypersensitivity also developed in SNL but not sham rats, evidenced by enhanced nociceptive behaviors induced by placement on a cold plate (6 degrees C) or application of icilin (a transient receptor potential M8 (TRPM8)/transient receptor potential A1 (TRPA1) receptor agonist) to nerve-injured hind paws. Single fiber recordings demonstrated that the mean conduction velocities of intact L4 cutaneous A delta- and C-fibers were not different between naive and SNL rats; however, mechanical thresholds of the A delta- but not the C-fibers were significantly decreased in SNL compared with naive. There was a higher prevalence of C-mechanoheat-cold (CMHC) fibers in SNL compared with naive, but the overall percentage of cold-sensitive C-fibers was not significantly increased compared with naive. This was in contrast to the numerous changes in A delta-fibers: the percentage of L4 cold sensitive A delta-, but not C-fibers, was significantly increased, the percentage of L4 icilin-sensitive A delta-, but not C-fibers, was significantly increased, the icilin-induced activity of L4 A delta-, but not C-fibers, was significantly increased. Icilin-induced activity was blocked by the TRPA1 antagonist Ruthenium Red. The results indicate plasticity in both A delta- and C-uninjured fibers, but A delta fibers appear to provide a major contribution to cold hypersensitivity in neuropathic rats.

Expression of the vesicular glutamate transporters-1 and -2 in adult mouse dorsal root ganglia and spinal cord and their regulation by nerve injury

The expression of two vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, was studied with immunohistochemistry in lumbar dorsal root ganglia (DRGs), the lumbar spinal cord and the skin of the adult mouse. About 12% and 65% of the total number of DRG neuron profiles (NPs) expressed VGLUT1 and VGLUT2, respectively. VGLUT1-immunoreactive (IR) NPs were usually medium- to large-sized, in contrast to a majority of small- or medium-sized VGLUT2-IR NPs. Most VGLUT1-IR NPs did not coexpress calcitonin gene-related peptide (CGRP) or bound isolectin B4 (IB4). In contrast, approximately 31% and approximately 42% of the VGLUT2-IR DRG NPs were also CGRP-IR or bound IB4, respectively. Conversely, virtually all CGRP-IR and IB4-binding NPs coexpressed VGLUT2. Moderate colocalization between VGLUT1 and VGLUT2 was also observed. Sciatic nerve transection induced a decrease in the overall number of VGLUT1- and VGLUT2-IR NPs (both ipsi- and contralaterally) and, in addition, a parallel, unilateral increase of VGLUT2-like immunoreactivity (LI) in a subpopulation of mostly small NPs. In the dorsal horn of the spinal cord, strong VGLUT1-LI was detected, particularly in deep dorsal horn layers and in the ventral horns. VGLUT2-LI was abundant throughout the gray spinal matter, 'radiating' into/from the white matter. A unilateral dorsal rhizotomy reduced VGLUT1-LI, while apparently leaving unaffected the VGLUT2-LI. Transport through axons for both VGLUTs was confirmed by their accumulation after compression of the sciatic nerve or dorsal roots. In the hind paw skin, abundant VGLUT2-IR nerve fibers were observed, sometimes associated with Merkel cells. Lower numbers of VGLUT1-IR fibers were also detected in the skin. Some VGLUT1-IR and VGLUT2-IR fibers were associated with hair follicles. Based on these data and those by Morris et al. [Morris JL, Konig P, Shimizu T, Jobling P, Gibbins IL (2005) Most peptide-containing sensory neurons lack proteins for exocytotic release and vesicular transport of glutamate. J Comp Neurol 483:1-16], we speculate that virtually all DRG neurons in adult mouse express VGLUTs and use glutamate as transmitter.

The tibial neuroma transposition (TNT) model of neuroma pain and hyperalgesia

Peripheral nerve injury may lead to the formation of a painful neuroma. In patients, palpating the tissue overlying a neuroma evokes paraesthesias/dysaesthesias in the distribution of the injured nerve. Previous animal models of neuropathic pain have focused on the mechanical hyperalgesia and allodynia that develops at a location distant from the site of injury and not on the pain from direct stimulation of the neuroma. We describe a new animal model of neuroma pain in which the neuroma was located in a position that is accessible to mechanical testing and outside of the innervation territory of the injured nerve. This allowed testing of pain in response to mechanical stimulation of the neuroma (which we call neuroma tenderness) independent of pain due to mechanical hyperalgesia. In the tibial neuroma transposition (TNT) model, the posterior tibial nerve was ligated and transected in the foot just proximal to the plantar bifurcation. Using a subcutaneous tunnel, the end of the ligated nerve was positioned just superior to the lateral malleolus. Mechanical stimulation of the neuroma produced a profound withdrawal behavior that could be distinguished from the hyperalgesia that developed on the hind paw. The neuroma tenderness (but not the hyperalgesia) was reversed by local lidocaine injection and by proximal transection of the tibial nerve. Afferents originating from the neuroma exhibited spontaneous activity and responses to mechanical stimulation of the neuroma. The TNT model provides a useful tool to investigate the differential mechanisms underlying the neuroma tenderness and mechanical hyperalgesia associated with neuropathic pain.

Spontaneous pain following spinal nerve injury in mice

Autotomy behavior is frequently observed in rats and mice in which the nerves of the hindlimb are severed, denervating the paw. This is the neuroma model of neuropathic pain. A large body of evidence suggests that this behavior reflects the presence of spontaneous dysesthesia and pain. In contrast, autotomy typically does not develop in partial nerve injury pain models, leading to the belief that these animals develop hypersensibility to applied stimuli (allodynia and hyperalgesia), but not spontaneous pain. We have modified the widely used Chung (spinal nerve ligation [SNL]) model of neuropathic pain in a way that retains the fundamental neural lesion, but eliminates nociceptive sensory cover of the paw. These animals performed autotomy. Moreover, the heritable across strains predisposition to spontaneous pain behavior in this new proximal denervation model (SNN) was highly correlated with pain phenotype in the neuroma model suggesting that the pain mechanism in the two models is the same. Relative reproducibility of strain predispositions across laboratories was verified. These data indicate that the neural substrate for spontaneous pain is present in the Chung-SNL model, and perhaps in the other partial nerve injury models as well, but that spontaneous pain is not expressed as autotomy in these models because there is protective nociceptive sensory cover.

Management of neuropathic orofacial pain

Current management of painful trigeminal neuropathies relies on pharmacological (topical and systemic), surgical, and complementary modalities. There is, however, a lack of quality research relating to the effectiveness of these modalities. In this review we analyze the available data that relates to the therapy of trigeminal neuralgia, postherpetic neuralgia, and posttraumatic neuropathies and provide clinical guidelines. The review focuses on medical management, as well as surgical and other interventions for painful neuropathies.

Voltage-gated Na+channels in neuropathic pain

Neuropathic pain occurs as a result of some form of injury to the nervous system. Although the basis of the disease remains to be fully elucidated, numerous studies have suggested a major role for ion channels in the pathogenesis of neuropathic pain. As Na+ channels play a fundamental role in not only the generation but also in the conduction of an action potential, they have received considerable attention in the aetiology of pain sensation and have become important pharmacological targets. In this review, the authors discuss the importance of specific Na+ channel isoforms in the pathophysiology of neuropathic pain and the present use of Na+ channel antagonists in the treatment of neuropathic pain.

The role of uninjured C-afferents and injured afferents in the generation of mechanical hypersensitivity after partial peripheral nerve injury in the rat

This study was performed to determine which of uninjured lumbar 4 (L4) C-afferents and injured L5 afferents was important for the generation of mechanical hypersensitivity following L5 spinal nerve ligation-and-cut (SNLC, modified spinal nerve ligation) in the rat. The mechanical hypersensitivity established following L5 SNLC was completely abolished 6 weeks after local capsaicin treatment of the sciatic nerve or L4 spinal nerve. At this stage, a substantial number of capsaicin-sensitive C-afferents were eliminated without any loss of A-afferents in the L4 spinal segment, suggesting that the capsaicin-sensitive L4 C-afferents are a major contributor to L5 SNLC-produced mechanical hypersensitivity. The peripheral terminals of L4 C-afferents are active in maintaining mechanical hypersensitivity, even long after L5 SNLC. When capsaicin-sensitive L4 C-afferents were previously eliminated, the induction of L5 SNLC-produced hypersensitivity was partly prevented. Thus, capsaicin-sensitive L4 C-afferents are crucial for the induction and maintenance of mechanical hypersensitivity in the L5 SNLC model. Also, when capsaicin-sensitive L4 C-afferents were previously eliminated, L5 SNLC still produced a partial mechanical hypersensitivity for a 1- to 2-week maintenance period with a several-day delay. This mild hypersensitivity was prevented by the previous L5 dorsal rhizotomy, implying an involvement of inputs from injured L5 afferents in the maintenance of hypersensitivity at the earlier stage. The results suggest that uninjured C-afferents, most likely C-polymodal nociceptors, are necessary for the induction and maintenance of neuropathic pain, and that afferent inputs, presumably from injured Abeta-fibers, also contribute to the maintenance at an earlier stage.

Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

Calcium-activated potassium channels regulate AHP and excitability in neurons. Since we have previously shown that axotomy decreases I(Ca) in DRG neurons, we investigated the association between I(Ca) and K((Ca)) currents in control medium-sized (30-39 microM) neurons, as well as axotomized L5 or adjacent L4 DRG neurons from hyperalgesic rats following L5 SNL. Currents in response to AP waveform voltage commands were recorded first in Tyrode's solution and sequentially after: 1) blocking Na(+) current with NMDG and TTX; 2) addition of K((Ca)) blockers with a combination of apamin 1 microM, iberiotoxin 200 nM, and clotrimazole 500 nM; 3) blocking remaining K(+) current with the addition of 4-AP, TEA-Cl, and glibenclamide; and 4) blocking I(Ca) with cadmium. In separate experiments, currents were evoked (HP -60 mV, 200 ms square command pulses from -100 to +50 mV) while ensuring high levels of activation of I(K(Ca)) by clamping cytosolic Ca(2+) concentration with pipette solution in which Ca(2+) was buffered to 1 microM. This revealed I(K(Ca)) with components sensitive to apamin, clotrimazole and iberiotoxin. SNL decreases total I(K(Ca)) in axotomized (L5) neurons, but increases total I(K(Ca)) in adjacent (L4) DRG neurons. All I(K(Ca)) subtypes are decreased by axotomy, but iberiotoxin-sensitive and clotrimazole-sensitive current densities are increased in adjacent L4 neurons after SNL. In an additional set of experiments we found that small-sized control DRG neurons also expressed iberiotoxin-sensitive currents, which are reduced in both axotomized (L5) and adjacent (L4) neurons.

CONCLUSIONS

Axotomy decreases I(K(Ca)) due to a direct effect on K((Ca)) channels. Axotomy-induced loss of I(Ca) may further potentiate current reduction. This reduction in I(K(Ca)) may contribute to elevated excitability after axotomy. Adjacent neurons (L4 after SNL) exhibit increased I(K(Ca)) current.

Proteomics study of neuropathic and nonneuropathic dorsal root ganglia: altered protein regulation following segmental spinal nerve ligation injury

Peripheral nerve injury is often followed by the development of severe neuropathic pain. Nerve degeneration accompanied by inflammatory mediators is thought to play a role in generation of neuropathic pain. Neuronal cell death follows axonal degeneration, devastating a vast number of molecules in injured neurons and the neighboring cells. Because we have little understanding of the cellular and molecular mechanisms underlying neuronal cell death triggered by nerve injury, we conducted a proteomics study of rat 4th and 5th lumbar (L4 and L5) dorsal root ganglion (DRG) after L5 spinal nerve ligation. DRG proteins were displayed on two-dimensional gels and analyzed through quantitative densitometry, statistical validation of the quantitative data, and peptide mass fingerprinting for protein identification. Among approximately 1,300 protein spots detected on each gel, we discovered 67 proteins that were tightly regulated by nerve ligation. We find that the injury to primary sensory neurons turned on multiple cellular mechanisms critical for the structural and functional integrity of neurons and for the defense against oxidative damage. Our data indicate that the regulation of metabolic enzymes was carefully orchestrated to meet the altered energy requirement of the DRG cells. Our data also demonstrate that ligation of the L5 spinal nerve led to the upregulation in the L4 DRG of the proteins that are highly expressed in embryonic sensory neurons. To understand the molecular mechanisms underlying neuropathic pain, we need to comprehend such dynamic aspect of protein modulations that follow nerve injury.

Pathophysiology of pain

The processing and interpretation of pain signals is a complex process that entails excitation of peripheral nerves, local interactions within the spinal dorsal horn, and the activation of ascending and descending circuits that comprise a loop from the spinal cord to supraspinal structures and finally exciting nociceptive inputs at the spinal level. Although the "circuits" described here appear to be part of normal pain processing, the system demonstrates a remarkable ability to undergo neuroplastic transformations when nociceptive inputs are extended over time, and such adaptations function as a pronociceptive positive feedback loop. Manipulations directed to disrupt any of the nodes of this pain facilitatory loop may effectively disrupt the maintenance of the sensitized pain state and diminish or abolish neuropathic pain. Understanding the ascending and descending pain facilitatory circuits may provide for the design of rational therapies that do not interfere with normal sensory processing.

Importance of hyperexcitability of DRG neurons in neuropathic pain

A number of good animal models have been developed in recent years that provide insights into the mechanisms of neuropathic pain. It now becomes evident that there are two separate peripheral components influencing neuropathic pain: one dependent on the hyperexcitability of axotomized dorsal root ganglion (DRG) neurons and the other independent of this hyperexcitability. The purpose of this review is to consider one of these components, the hyperexcitability of axotomized DRG neurons, as one of the important mechanisms underlying neuropathic pain. Several hours after nerve lesions, some axotomized DRG neurons become hyperexcitable and begin to show ongoing discharges that last many days or weeks. These ectopic discharges then enter the spinal cord and induce central sensitization, the underlying central mechanism for the generation of pain and allodynia. Although the exact causes of the development of hyperexcitability and ectopic discharges are not clear, various ion channels seem to play important roles, particularly sodium channels. In addition, important modulatory factors for ectopic discharges are purinergic and adrenergic components of the sympathetic nervous system. These findings suggest that manipulating sodium channels and/or adrenergic and purinergic receptors on axotomized DRG cells may give neuropathic pain sufferers some relief that is not available from present treatment regimens.

Mechanisms of neuropathic pain

Neuropathic pain refers to pain that originates from pathology of the nervous system. Diabetes, infection (herpes zoster), nerve compression, nerve trauma, "channelopathies," and autoimmune disease are examples of diseases that may cause neuropathic pain. The development of both animal models and newer pharmacological strategies has led to an explosion of interest in the underlying mechanisms. Neuropathic pain reflects both peripheral and central sensitization mechanisms. Abnormal signals arise not only from injured axons but also from the intact nociceptors that share the innervation territory of the injured nerve. This review focuses on how both human studies and animal models are helping to elucidate the mechanisms underlying these surprisingly common disorders. The rapid gain in knowledge about abnormal signaling promises breakthroughs in the treatment of these often debilitating disorders.

Dissociation of dorsal root ganglion neurons induces hyperexcitability that is maintained by increased responsiveness to cAMP and cGMP

Injury or inflammation affecting sensory neurons in dorsal root ganglia (DRG) causes hyperexcitability of DRG neurons that can lead to spontaneous firing and neuropathic pain. Recent results indicate that after chronic compression of DRG (CCD treatment), both hyperexcitability of neurons in intact DRG and behaviorally expressed hyperalgesia are maintained by concurrent activity in cAMP-protein kinase A (PKA) and cGMP-protein kinase G (PKG) signaling pathways. We report here that when tested under identical conditions, dissociation produces a pattern of hyperexcitability in small DRG neurons similar to that produced by CCD treatment, manifest as decreased action potential (AP) current threshold, increased AP duration, increased repetitive firing to depolarizing pulses, increased spontaneous firing and resting depolarization. A novel feature of this hyperexcitability is its early expression-as soon as testing can be conducted after dissociation (approximately 2 h). Both forms of injury increase the electrophysiological responsiveness of the neurons to activation of cAMP-PKA and cGMP-PKG pathways as indicated by enhancement of hyperexcitability by agonists of these pathways in dissociated or CCD-treated neurons but not in control neurons. Although inflammatory signals are known to activate cAMP-PKA pathways, dissociation-induced hyperexcitability is unlikely to be triggered by signals released from inflammatory cells recruited to the DRG because of insufficient time for recruitment during the dissociation procedure. Inhibition by specific antagonists indicates that continuing activation of cAMP-PKA and cGMP-PKG pathways is required to maintain hyperexcitability after dissociation. The reduction of hyperexcitability by blockers of adenylyl cyclase and soluble guanylyl cyclase after dissociation suggests a continuing release of autocrine and/or paracrine factors from dissociated neurons and/or satellite cells, which activate both cyclases and help to maintain acute, injury-induced hyperexcitability of DRG neurons.

Increased sodium channel immunofluorescence at myelinated and demyelinated sites following an inflammatory and partial axotomy lesion of the rat infraorbital nerve

The localization of sodium channels (NaChs) change following nerve lesions and this change may contribute to the development of increased pain states. Here we examine the change in distribution of NaChs within the rat infraorbital nerve (ION) two weeks after a combined inflammatory/partial axotomy lesion that results in behavior showing increased sensitivity to mechanical stimuli. Sections from experimental and normal control IONs were double-stained for indirect immunofluorescence using an antibody that identifies all NaCh isoforms and caspr-antibody to identify nodes of Ranvier, and a confocal microscope z-series of optically sectioned images were then obtained. ImageJ (NIH) software was used to quantify the area of pixels showing maximum NaCh intensity within both caspr and non-caspr associated accumulations. Analysis showed that the lesioned IONs had many more split nodes, heminodes and caspr-negative "naked" accumulations, a significantly increased area of NaCh staining within typical nodes and "naked" accumulations, as well as an increased density and size of significant accumulations when compared to normal IONs. This study demonstrates a dramatic redistribution and increased immunofluorescence of NaChs especially at myelinated and demyelinated sites in fibers located just proximal to the lesion. The remodeling of NaChs seen in this study may represent an important event associated with the development of increased nerve excitability after lesions.

Inflammatory hyperalgesia: a role for the C-fiber sensory neuron cell body?

Peripheral nerve injury increases the chemosensitivity and excitability of injured afferents, resulting in ectopic activity arising from within dorsal root ganglia. Studies of dissociated sensory ganglion neurons in vitro suggest afferent somata might be sensitized by persistent inflammation. However, it is unknown whether this inflammation-induced sensitization is manifest in somata within the intact ganglia. To explore this possibility, intracellular electrophysiologic recording was used with a sciatic nerve-L4-dorsal root ganglia preparation to compare excitability and chemosensitivity of cutaneous C-fiber somata from control and inflamed rats. Cutaneous afferents were identified with the retrograde dye DiI. Excitability was assessed before and after the application of inflammatory soup (IS) containing bradykinin, serotonin, and prostaglandin E2 all at a pH of 7.0. Persistent inflammation decreased the excitability of cutaneous afferents in intact ganglia and had no significant influence on the magnitude of IS-induced increase in excitability. Opposite to the effects observed in intact ganglia, excitability was greater in dissociated cutaneous nociceptors obtained from inflamed rats, although the magnitude of the IS-induced increase in excitability was not significantly affected by inflammation. These results suggest that the cell bodies of putative cutaneous nociceptors in the intact ganglia contribute minimally to pain and hyperalgesia associated with persistent inflammation.

PERSPECTIVE

Results of the present study suggest that inflammation-induced changes in afferent somata are minimal. However, they also suggest that inflammatory mediator-induced increase in the excitability of sensory neuron somata might contribute to global changes in nociception observed under high systemic inflammatory mediator loads.

Descending facilitation from the rostral ventromedial medulla maintains nerve injury-induced central sensitization

Nerve injury can produce hypersensitivity to noxious and normally innocuous stimulation. Injury-induced central (i.e. spinal) sensitization is thought to arise from enhanced afferent input to the spinal cord and to be critical for expression of behavioral hypersensitivity. Descending facilitatory influences from the rostral ventromedial medulla have been suggested to also be critical for the maintenance, though not the initiation, of experimental neuropathic pain. The possibility that descending facilitation from the rostral ventromedial medulla is required for the maintenance of central sensitization was examined by determining whether ablation of mu-opioid receptor-expressing cells within the rostral ventromedial medulla prevented the enhanced expression of repetitive touch-evoked FOS within the spinal cord of animals with spinal nerve ligation injury as well as nerve injury-induced behavioral hypersensitivity. Rats received a single microinjection of vehicle, saporin, dermorphin or dermorphin-saporin into the rostral ventromedial medulla and 28 days later, underwent either sham or spinal nerve ligation procedures. Animals receiving rostral ventromedial medulla pretreatment with vehicle, dermorphin or saporin that were subjected to spinal nerve ligation demonstrated both thermal and tactile hypersensitivity, and showed significantly increased expression of touch-evoked FOS in the dorsal horn ipsilateral to nerve injury compared with sham-operated controls at days 3, 5 or 10 post-spinal nerve ligation. In contrast, nerve-injured animals pretreated with dermorphin-saporin showed enhanced behaviors and touch-evoked FOS expression in the spinal dorsal horn at day 3, but not days 5 and 10, post-spinal nerve ligation when compared with sham-operated controls. These results indicate the presence of nerve injury-induced behavioral hypersensitivity associated with nerve injury-induced central sensitization. Further, the results demonstrate the novel concept that once initiated, maintenance of nerve injury-induced central sensitization in the spinal dorsal horn requires descending pain facilitation mechanisms arising from the rostral ventromedial medulla.

Response to spinal cord stimulation in variants of the spared nerve injury pain model

Spinal cord stimulation (SCS) is a treatment given to patients with drug-resistant neuropathic pain, in particular pain resulting from peripheral nerve injury. However, the reasons why some patients develop neuropathic pain and why SCS is not effective in all patients with this chronic pain are not fully understood. The present study compares the response to SCS and the yield of neuropathic animals in variants of the spared nerve injury (SNI) model introduced by Decosterd and Woolf (I. Decosterd, C.J. Woolf, Spared nerve injury: an animal model of persistent peripheral neuropathic pain, Pain 87 (2000) 149-158). Sprague-Dawley rats were prepared with various types of lesions of different branches of the sciatic nerve and then tested for paw mechanical hypersensitivity. A miniature electrode system for SCS was implanted at the T10-T11 vertebral level. Stimulation was applied in awake, freely moving animals with parameters comparable to those employed clinically. Suppression of paw hypersensitivity was considered a positive response to SCS. The incidence of mechanical hypersensitivity ("allodynia") in the different models was: SNI 53%; peroneal axotomy 45%; tibial axotomy 68%; tibial tight ligation 73% and partial tibial tight ligation 50%. "Mirror phenomena" with contralateral paw hypersensitivity was present in about 20% of the animals. The response to SCS differed between models with the lowest response rate in the original SNI model (8%) while the others demonstrated rates in the order of 40-50%. There was a tendency that the efficacy of SCS in suppressing allodynia was inversely related to the severity of hypersensitivity. In conclusion, modifications of the SNI model provide a reproducible incidence of neuropathic hypersensitivity and an increased responsiveness to SCS. These variants may prove suitable for future research on the mechanisms involved in pain relief with SCS.

The Role of Sodium Channels in Chronic Inflammatory and Neuropathic Pain

Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers.

PERSPECTIVE

Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Inhibitory M2 muscarinic receptors are upregulated in both axotomized and intact small diameter dorsal root ganglion cells after peripheral nerve injury

Acetylcholine reduces nociceptive input in part by activating inhibitory M2 muscarinic receptors on primary sensory neurons, and acetylcholinesterase inhibitors and muscarinic agonists produce analgesia in humans and animals. M2 muscarinic receptors are upregulated in animals with diabetic neuropathy, but their level of expression and function after peripheral nerve injury has not been previously examined. This study tested, using intracellular Ca(2+) response to membrane depolarization, the effect of the M2 muscarinic receptor agonist bethanechol on individual dorsal root ganglion cells from normal and L5-6 spinal nerve-ligated rats, followed by M2 muscarinic receptor immunostaining. We also examined functional transient receptor potential for vanilloids-1 activity by determining intracellular Ca(2+) response evoked by capsaicin in M2 muscarinic receptor immunoreactive cells. In normal dorsal root ganglion cells, bethanechol inhibited the Ca(2+) response in a concentration-related fashion, and this inhibition was blocked by the M2 muscarinic receptor antagonist gallamine. Cells expressing M2 muscarinic receptors by immunostaining were significantly inhibited by bethanechol, whereas those lacking positive staining were not. The proportion of studied dorsal root ganglion neurons with positive M2 muscarinic receptor staining increased significantly in the injured ipsilateral L5-6 and the uninjured ipsilateral L4 ganglia, but not in the contralateral dorsal root ganglion neurons compared with normals. In contrast, the proportion of neurons responding to capsaicin significantly decreased in the injured ipsilateral L5-6 dorsal root ganglion cells. These results suggest that inhibitory M2 muscarinic receptors are upregulated in small- and medium-sized axotomized dorsal root ganglion neurons and their uninjured neighbors following nerve injury, and may represent an appropriate target for analgesia in this setting.

Spontaneous pain, both neuropathic and inflammatory, is related to frequency of spontaneous firing in intact C-fiber nociceptors

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL). To determine whether SFL is caused by spontaneous firing in nociceptive neurons, we studied the following groups of rats: (1) untreated; (2) spinal nerve axotomy (SNA), L5 SNA 1 week earlier; (3) mSNA (modified SNA), SNA plus loose ligation of the adjacent L4 spinal nerve with inflammation-inducing chromic gut; and (4) CFA (complete Freund's adjuvant), intradermal complete Freund's adjuvant-induced hindlimb inflammation 1 and 4 d earlier. In all groups, recordings of SFL and of spontaneous activity (SA) in ipsilateral dorsal root ganglion (DRG) neurons (intracellularly) were made. Evoked pain behaviors were measured in nerve injury (SNA/mSNA) groups. Percentages of nociceptive-type C-fiber neurons (C-nociceptors) with SA increased in intact L4 but not axotomized L5 DRGs in SNA and mSNA (to 35%), and in L4/L5 DRGs 1-4 d after CFA (to 38-25%). SFL occurred in mSNA but not SNA rats. It was not correlated with mechanical allodynia, extent of L4 fiber damage [ATF3 (activation transcription factor 3) immunostaining], or percentage of L4 C-nociceptors with SA. However, L4 C-nociceptors with SA fired faster after mSNA (1.8 Hz) than SNA (0.02 Hz); estimated L4 total firing rates were approximately 5.0 and approximately 0.6 kHz, respectively. Similarly, after CFA, faster L4 C-nociceptor SA after 1 d was associated with SFL, whereas slower SA after 4 d was not. Thus, inflammation causes L4 C-nociceptor SA and SFL. Overall, SFL was related to SA rate in intact C-nociceptors. Both L5 degeneration and chromic gut cause inflammation. Therefore, both SA and SFL/spontaneous pain after nerve injury (mSNA) may result from cumulative neuroinflammation.

Genetic impairment of interleukin-1 signaling attenuates neuropathic pain, autotomy, and spontaneous ectopic neuronal activity, following nerve injury in mice

Peripheral nerve injury may lead to neuropathic pain, which is often associated with mechanical and thermal allodynia, ectopic discharge of from injured nerves and from the dorsal root ganglion neurons, and elevated levels of proinflammatory cytokines, particularly interleukin-1 (IL-1). In the present study, we tested the role of IL-1 in neuropathic pain models using two mouse strains impaired in IL-1 signaling: Deletion of the IL-1 receptor type I (IL-1rKO) and transgenic over-expression of the IL-1 receptor antagonist (IL-1raTG). Neuropathy was induced by cutting the L5 spinal nerve on one side, following which mechanical and thermal pain sensitivity was measured. Wild-type (WT) mice and the parent strains developed significant allodynia and hyperalgesia in the hind-paw ipsilateral to the injury compared with the contralateral hind-paw. The mutant strains, however, did not display decreased pain threshold in either hind-paw. Pain behavior was also assessed by cutting the sciatic and saphenous nerves and measuring autotomy scores. WT mice developed progressive autotomy, beginning at 7 days post-injury, whereas the mutant strains displayed delayed onset of autotomy and markedly reduced severity of the autotomy score. Electrophysiological assessment revealed that in WT mice a significant proportion of the dorsal root axons exhibited spontaneous ectopic activity at 1, 3, and 7 days following spinal nerve injury, whereas in IL-1rKO and IL-1raTG mice only a minimal number of axons exhibited such activity. Taken together, these results suggest that IL-1 signaling plays an important role in neuropathic pain and in the altered neuronal activity that underlies its development.

cAMP and cGMP Contribute to Sensory Neuron Hyperexcitability and Hyperalgesia in Rats With Dorsal Root Ganglia Compression

Numerous studies have implicated the cAMP-protein kinase A (PKA) pathway in producing hyperexcitability of dorsal root ganglia (DRG) sensory neurons under conditions associated with pain. Evidence is presented for roles of both the cAMP-PKA and cGMP-protein kinase G (PKG) pathways in maintaining neuronal hyperexcitability and behavioral hyperalgesia in a neuropathic pain model: chronic compression of the DRG (CCD treatment). Lumbar DRGs were compressed by a steel rod inserted into the intervertebral foramen. Thermal hyperalgesia was revealed by shortened latencies of foot withdrawal to radiant heat. Intracellular recordings were obtained in vitro from lumbar ganglia after in vivo DRG compression. Activators of the cAMP-PKA pathway, 8-Br-cAMP and Sp-cAMPS, and of the cGMP-PKG pathway, 8-Br-cGMP and Sp-cGMPS, increased the hyperexcitability of DRG neurons already produced by CCD treatment, as shown by further decreases in action potential threshold and increased repetitive discharge during depolarization. The adenylate cyclase inhibitor, SQ22536, the PKA antagonist, Rp-cAMPS, the guanylate cyclase inhibitor, ODQ, and the PKG inhibitor, Rp-8-pCPT-cGMPS, reduced the hyperexcitability of CCD DRG neurons. In vivo application of PKA and PKG antagonists transiently depressed behavioral hyperalgesia induced by CCD treatment. Unexpectedly, application of these agonists and antagonists to ganglia of naïve, uninjured animals had little effect on electrophysiological properties of DRG neurons and no effect on foot withdrawal, suggesting that sensitizing actions of these pathways in the DRG are enabled by prior injury or stress. The only effect observed in uncompressed ganglia was modest depolarization of DRG neurons by PKA and PKG agonists. CCD treatment also depolarized DRG neurons, but CCD-induced depolarization was not affected by agonists or antagonists of these pathways.

Hyperexcitability of axotomized and neighboring unaxotomized sensory neurons is reduced days after perineural clonidine at the site of injury

Hyperexcitability after peripheral nerve injury occurs in axotomized and neighboring unaxotomized dorsal root ganglion (DRG) neurons and contributes to hypersensitivity. Previous studies have focused on proximal nerve injury and have not examined unaxotomized neurons innervating the site of sensory testing. The current study used a distal nerve injury (partial sciatic nerve ligation [PSNL]), and identified, using fluorescent tracers, axotomized and unaxotomized neurons innervating the site of hypersensitivity. We hypothesized that reduced hypersensitivity after perineural clonidine was associated with reduced hyperexcitability of DRG neurons. Rats underwent sham or PSNL surgery, followed 2 wk later by a single injection at the injury site of clonidine or saline. PSNL, but not sham surgery, reduced hindpaw mechanical withdrawal threshold, and clonidine, but not saline, partially reversed this effect 3 days after injection. Intracellular recording of neurons in whole DRG demonstrated similar changes in membrane properties and excitability in unaxotomized and axotomized neurons after PSNL compared with sham surgery, primarily depolarized resting membrane potential, reduced rheobase, presence of oscillations, and capability to fire repetitively. Most of these changes were present in small-, medium-, and large-diameter neurons. Perineural clonidine 3 days later significantly reversed many of these effects, whereas saline was without effect. We speculate that perineural clonidine reduces signals, likely proinflammatory cytokines and prostaglandins produced during Wallerian degeneration after nerve injury, which drive changes in ion channel expression in DRG somata leading to hyperexcitability and hypersensitivity.

Mechanisms underlying enhanced P2X receptor-mediated responses in the neuropathic pain state

P2X3 and P2X2/3 receptors in dorsal root ganglia (DRG) appear to participate in producing nociceptive responses after nerve injury. However, the mechanisms underlying the receptor-mediated nociception in the neuropathic state remain unclear. Using spared nerve injury (SNI) rats, we found that allodynic and nocifensive (flinch) behavioral responses developed after injury can be reversed by P2X receptor antagonists, indicating an involvement of P2X receptors. Immunocytochemical studies revealed that P2X3 receptors are expressed in small and medium but rarely in large DRG neurons of both normal and SNI rats. Thus, contrary to the conventional view that only large A beta cells mediate allodynia, small and medium cells are intimately involved in P2X3 receptor-mediated allodynia. Measuring ATP levels in the subcutaneous space of the rat paw, we showed that ATP release does not change after SNI. On the other hand, the P2X receptor agonist, alpha beta-methylene ATP produces 3.5-fold larger flinch responses at a 8.0-fold lower dose. Thus, sensitization of P2X3 receptors rather than a change in ATP release is responsible for the neuropathic pain behaviors. We further demonstrated that sensitization of P2X3 receptors arises from an increase in receptor function. ATP-induced P2X3 receptor-mediated currents in DRG neurons is 2.5-fold larger after SNI. The expression of P2X3 receptors on the cell membrane is significantly enhanced while the total expression of P2X3 receptors remained unchanged. Thus, the enhancement of trafficking of P2X3 receptors is likely an important mechanism contributing to the increase in receptor function after nerve injury.

Block of peripheral nerve sodium channels selectively inhibits features of neuropathic pain in rats

Several sodium channel blockers are used clinically to treat neuropathic pain. However, many patients fail to achieve adequate pain relief from these highly brain-penetrant drugs because of dose-limiting central nervous system side effects. Here, we describe the functional properties of trans-N-{[2'-(aminosulfonyl)biphenyl-4-yl]methyl}-N-methyl-N'-[4-(trifluoromethoxy)benzyl]cyclopentane-1,2-dicarboxamide (CDA54), a peripherally acting sodium channel blocker. In whole-cell electrophysiological assays, CDA54 blocked the inactivated states of hNa(V)1.7 and hNa(V)1.8, two channels of the peripheral nervous system implicated in nociceptive transmission, with affinities of 0.25 and 0.18 microM, respectively. CDA54 displayed similar affinities for the tetrodotoxin-resistant Na+ current in small-diameter mouse dorsal root ganglion neurons. Peripheral nerve injury causes spontaneous electrical activity in normally silent sensory neurons. CDA54 inhibited these injury-induced spontaneous action potentials at concentrations 10-fold lower than those required to block normal A- and C-fiber conduction. Consistent with the selective inhibition of injury-induced firing, CDA54 (10 mg/kg p.o.) significantly reduced behavioral signs of neuropathic pain in two nerve injury models, whereas the same dose of CDA54 did not affect acute nociception or motor coordination. In anesthetized dogs, CDA54, at plasma concentrations of 6.7 microM, had no effect on cardiac electrophysiological parameters including conduction. Thus, the peripheral nerve sodium channel blocker CDA54 selectively inhibits sensory nerve signaling associated with neuropathic pain.

Activation of nuclear factor-kappaB via endogenous tumor necrosis factor alpha regulates survival of axotomized adult sensory neurons

Embryonic dorsal root ganglion (DRG) neurons die after axonal damage in vivo, and cultured embryonic DRG neurons require exogenous neurotrophic factors that activate the neuroprotective transcription factor nuclear factor-kappaB (NF-kappaB) for survival. In contrast, adult DRG neurons survive permanent axotomy in vivo and in defined culture media devoid of exogenous neurotrophic factors in vitro. Peripheral axotomy in adult rats induces local accumulation of the cytokine tumor necrosis factor alpha (TNFalpha), a potent activator of NF-kappaB activity. We tested the hypothesis that activation of NF-kappaB stimulated by endogenous TNFalpha was required for survival of axotomized adult sensory neurons. Peripheral axotomy of lumbar DRG neurons by sciatic nerve crush induced a very rapid (within 2 h) and significant elevation in NF-kappaB-binding activity. This phenomenon was mimicked in cultured neurons in which there was substantial NF-kappaB nuclear translocation and a significant rise in NF-kappaB DNA-binding activity after plating. Inhibitors of NF-kappaB (SN50 or NF-kappaB decoy DNA) resulted in necrotic cell death of medium to large neurons (> or =40 microm) within 24 h (60 and 75%, respectively), whereas inhibition of p38 and mitogen-activated protein/extracellular signal-regulated kinase did not effect survival. ELISA revealed that these cultures contained TNFalpha, and exposure to an anti-TNFalpha antibody inhibited NF-kappaB DNA-binding activity by approximately 35% and killed approximately 40% of medium to large neurons within 24 h. The results show for the first time that cytokine-mediated activation of NF-kappaB is a component of the signaling pathway responsible for maintenance of adult sensory neuron survival after axon damage.

Mechanical and heat sensitization of cutaneous nociceptors in rats with experimental peripheral neuropathy

This study examined whether or not the properties of cutaneous nociceptive fibers are altered in the neuropathic state by comparing lumbars 5 and 6 spinal nerve ligation (SNL) rats with sham-operated controls. The rats with the unilateral SNL developed mechanical allodynia in the ipsilateral hind limb, whereas the sham group did not. Two to 5 weeks after the neuropathic or sham surgery, rats were subjected to single fiber-recording experiments to examine the properties of afferent fibers in the sural and plantar nerves. A total of 224 afferents in the C- and Adelta-ranges were characterized in the neuropathic and sham groups. Spontaneous activity was observed in 16 of 155 fibers in the neuropathic group and one of 69 fibers in the sham group. The response threshold of both the C- and Adelta-fibers to mechanical stimuli was lower in the neuropathic group than the sham group. The afferent fibers responsive to heat stimuli were all C-fibers, and none were Adelta-fibers. The response threshold of the C-fibers to the heat stimuli was lower in the neuropathic group than the sham group. The magnitude of the responses of both C- and Adelta-fibers to the suprathreshold intensity of the mechanical stimulus was greater in the neuropathic group than the sham group. However, the magnitude of the responses of C-fibers to the suprathreshold intensity of the heat stimulus in the neuropathic group was not different from that in the sham group. These results suggest that after a partial peripheral nerve injury, the nociceptors on the skin supplied by an uninjured nerve become sensitized to both mechanical and heat stimuli. This nociceptor sensitization can contribute to neuropathic pain.

Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation

To investigate whether activation of mitogen-activated protein kinase (MAPK) in damaged and/or undamaged primary afferents participates in neuropathic pain after partial nerve injury, we examined the phosphorylation of extracellular signal-regulated protein kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK) in the L4 and L5 dorsal root ganglion (DRG) in the L5 spinal nerve ligation (SNL) model. We first confirmed, using activating transcription factor 3 and neuropeptide Y immunoreactivity, that virtually all L4 DRG neurons are spared from axotomy in this model. In the injured L5 DRG, the L5 SNL induced the activation of ERK, p38, and JNK in different populations of DRG neurons. In contrast, in the uninjured L4 DRG, the L5 SNL induced only p38 activation in tyrosine kinase A-expressing small- to medium-diameter neurons. Intrathecal ERK, p38, and JNK inhibitor infusions reversed SNL-induced mechanical allodynia, whereas only p38 inhibitor application attenuated SNL-induced thermal hyperalgesia. Furthermore, the L5 dorsal rhizotomy did not prevent SNL-induced thermal hyperalgesia. We therefore hypothesized that p38 activation in the uninjured L4 DRG might be involved in the development of heat hypersensitivity in the L5 SNL model. In fact, the treatment of the p38 inhibitor and also anti-nerve growth factor reduced SNL-induced upregulation of brain-derived neurotrophic factor and transient receptor potential vanilloid type 1 expression in the L4 DRG. Together, our results demonstrate that the L5 SNL induces differential activation of MAPK in injured and uninjured DRG neurons and, furthermore, that MAPK activation in the primary afferents may participate in generating pain hypersensitivity after partial nerve injury.

alpha2-adrenoceptors inhibit the intracellular Ca2+ response to electrical stimulation in normal and injured sensory neurons, with increased inhibition of calcitonin gene-related peptide expressing neurons after injury

Nerve injury resulting in chronic pain is associated with novel excitatory effects of norepinephrine on injured peripheral nerve terminals and their cell bodies, due to actions on alpha2-adrenoceptors. Paradoxically, alpha2-adrenoceptor agonists administered near peripheral terminals or their cell bodies results in analgesia, not pain. This study tested, using intracellular Ca2+ response to stimulation, the effects of alpha2-adrenoceptor agonists on injured sensory neurons and classified their neuronal phenotype. Dorsal root ganglion cells from normal and spinal nerve-ligated rats were dissociated and activated twice with electrical field stimulation, while measuring Fura-2 fluorescence. Cells were perfused between stimulations with vehicle or alpha2-adrenoceptor agonists alone or with antagonists. Cells were considered inhibited if the ratio of their peak Ca2+ response to the second stimulus divided by the first was less than the 2.5th percentile for vehicle controls. alpha2-, But not alpha1-adrenoceptor agonists inhibited the Ca2+ response in a concentration related fashion, and this inhibition was blocked by alpha2-adrenoceptor antagonists. Clonidine inhibited a similar percentage of cells in the normal and spinal nerve-ligated group. In both groups, the large majority of clonidine-inhibited cells stained for isolectin B4. Spinal nerve ligation resulted in a 4-10-fold increase in the percentage of clonidine inhibited cells which immunostained for calcitonin gene-related peptide. These data are consistent with the known inhibition of Ca2+ currents by alpha2-adrenoceptors and suggest that, at the level of intracellular Ca2+, the key determinant of neurotransmitter release, alpha2-adrenoceptors are inhibitory after nerve injury, not excitatory. There is a shift in phenotype of sensory neurons which are inhibited by clonidine after nerve injury, which may explain clonidine's increased potency in the treatment of neuropathic compared with acute pain.